The present invention relates to the delivery of liquid reactants to a combustion zone formed adjacent a burner assembly to create soot used in the manufacture of glass. More particularly, the present invention relates to a system and method of delivering liquid reactants to a combustion zone that avoids the premature solidification of the liquid reactants within the burner assembly.
While the invention is subject to a wide range of glass soot deposition applications, it is especially suited for use in producing soot for glass preforms used in the manufacture of optical waveguides, and will be particularly described in that connection.
Various processes are known in the art that involve the production of metal oxides from vaporous reactants. Such processes require a feedstock solution or precursor, a means of generating and transporting vapors of the feedstock solution (hereafter called vaporous reactants) and an oxidant to a conversion reaction site (also known as a soot reaction zone or combustion zone to those skilled in the art), and a means of catalyzing oxidation and combustion coincidentally to produce finely divided, spherical aggregates, called soot. This soot can be collected in any number of ways, ranging from capture in a collection chamber to deposition on a rotating mandrel. The collected soot may be simultaneously or subsequently heat treated to form a non-porous, transparent, high purity glass article. This process is usually carried out with specialized equipment having a unique arrangement of nozzles, injectors, burners and/or burner assemblies.
Much of the initial research that led to the development of such processes focused on the production of bulk silica. Selection of the appropriate feedstock was an important aspect of that work. Consequently, it was at that time determined that a material capable of generating a vapor pressure of between 200-300 millimeters of mercury (mm Hg) at temperatures below approximately 100xc2x0 C. would be useful for making such bulk silica. The high vapor pressure of silicon tetrachloride (SiCl4) suggested its usefulness as a convenient vapor source for soot generation and launched the discovery and use of a series of similar chloride-based feedstocks. This factor, more than any other is responsible for the presently accepted use of SiCl4, GeCl4, POCl3, and BCl3 as feedstock vapor sources.
Use of these and other halide-based feedstocks as vapor sources, however, does have its drawbacks. The predominate drawback being the formation of hydrochloric acid (HCl) as a by-product of oxidation. HCl is not only detrimental to the deposition substrates and the reaction equipment, but to the environment as well. Overcoming this drawback, amongst others, led to the use of halide-free compounds as precursors or feedstocks for the production of soot for optical waveguides.
Although use of halide-free silicon compounds as feedstocks for fused silica glass production, as described in U.S. Pat. Nos. 5,043,002 and 5,152,819, for example, avoids the formation of HCl, other problems remain, particularly when the soot is intended for the formation of optical waveguides. It has been found that, in the course of delivering a vaporized polyalkylsiloxane to the burner, high molecular weight species can be deposited as gels in the lines carrying the vaporous reactants to the burner, or within the burner itself. This leads to a reduction in the deposition rate of the soot that is subsequently consolidated to a blank from which an optical waveguide fiber is drawn. It also leads to imperfections in the blank that often produce defective and/or unusable optical waveguide fiber from the effected portions of the blank. An additional problem encountered while forming silica soot using siloxane feedstocks is the deposition of particulates having high molecular weights and high boiling points on the optical waveguide fiber blank. The build-up of these particulates results in xe2x80x9cdefectxe2x80x9d or xe2x80x9cclustered defectxe2x80x9d imperfections that adversely affect the optical and structural quality of optical waveguides formed using the silica soot.
Other feedstocks, some of which are, and others of which may be useful in forming soot for the manufacture of optical waveguides are not currently acceptable alternatives to the halide-based and halide-free feedstocks for delivery via vapor deposition. Materials such as salts and those known as rare-earth elements, for example, are extremely unstable as vapors and often decompose before they can be delivered in their vapor phase, or do not have sufficiently high vapor pressures to be vaporized at accessible temperatures.
Although it is often possible to deliver at least a percentage of these elements to the combustion zone as a vapor, it is technically very difficult. Elaborate systems incorporating expensive equipment are necessary to convert these elements to the vapor phase, and further, to deliver them to the combustion zone without leaving deposits in the lines leading to the burners and in the burners themselves. Moreover, if multiple elements are being delivered as vapors and a specific percentage of each is needed for the desired composition, it is difficult to control the delivery since different elements have different vapor pressures.
U.S. patent application Ser. No. 08/767,653, discloses that these and other limitations can be overcome by delivering a feedstock to an injector or burner in liquid form, atomizing the feedstock to form an aerosol containing fine droplets of the liquid feedstock, and converting the atomized liquid feedstock into soot at the combustion zone. Because the feedstock is delivered directly into the burner flame as a liquid rather than a vapor, the vapor pressure of tie feedstock is no longer a limiting factor in the formation of soot for use in the manufacture of optical waveguides The injectors, burners, and burner assemblies disclosed in U.S. patent application Ser. No. 08/767,653 and other currently pending applications rely on very small orifices to deliver the liquid in a fine stream for proper atomization. Because the orifices are so small, they are extremely susceptible to plugging. Even a small solid particle in the liquid being delivered can partially clog the orifice, which in turn adversely effects the soot deposition rate, and the homogeneity of the soot collected.
Although materials never before delivered to a combustion zone to form soot for the manufacture of glass can now be delivered in a liquid solution, many of these materials have inherent short-comings while in a liquid form. Most problematic is that many of these liquid materials quickly form solids when exposed to oxygen and/or water. Thus, any exposure to the air during liquid delivery of these reactants likely will result in the formation of solids, which clog the lines leading to the burners and the small orifices of the burners and the burner assemblies themselves. When the orifices become partially clogged, the flame, and thus the soot stream becomes non-uniform and the soot deposition rate suffers. As a result, the liquid delivery system must be shut down so that it can be cleaned. Such cleaning operations typically require partial disassembly of the burner assembly, which results in significant production down time.
In liquid delivery systems, plugging or clogging of the burner assembly orifices is particularly problematic during the start up and shut down stages of the liquid delivery cycle. During these periods, the liquid reactant tends to trickle or sputter out of the injector orifice. This occurs during the start up stage of the liquid delivery cycle before steady state pressure is available, and at the shut down stage of the liquid delivery cycle after steady state pressure is no longer available. These limited pressure stages result in significantly reduced liquid flow rates, which in turn can provide the exposure time necessary for the slow moving liquid to react with the air to form solids. Alternatively, these liquid feedstocks can leak and solidify on the burner face or within the burner head cavity, resulting in increased down time for cleaning or unclogging the burner. Because the liquid feedstocks can react with water in the air almost instantaneously, any amount of slow moving liquid feedstock within the injector or at the injector orifice can result in deposit of solids. The resultant partial plugging degrades burner performance for rate and quality of the soot produced, and complete plugging will stop soot deposition altogether.
There is a need therefore, for a system and method of delivering liquid feedstocks or precursors (hereinafter, xe2x80x9cliquid reactantsxe2x80x9d) through a burner assembly to form soot for the manufacture of glass that eliminates the premature solidification of the liquid reactants and therefore the plugging of burner assemblies in liquid delivery systems.
The present invention is directed to a system and method for delivering liquid reactants to a combustion zone adjacent a burner assembly of a liquid delivery system to produce soot for use in the manufacture of glass. In a liquid delivery system, the liquid reactant, capable of being converted by thermal oxidative decomposition to glass, is introduced directly into the combustion zone of a combustion burner; thereby forming finely divided amorphous soot. The amorphous soot is typically deposited on a receptor surface where, either substantially simultaneously with or subsequent to its deposition, the soot is consolidated into a body of fused glass. The body of glass may then be either used to make products directly from the fused body, or the fused body may be further treated, e.g., by forming an optical waveguide such as by drawing to make optical waveguide fiber as further described in, for example, U.S. patent application Ser. No. 08/574,961 entitled, xe2x80x9cMethod for Purifying Polyalkylsiloxanes and the Resulting Productsxe2x80x9d, the specification of which is hereby incorporated by reference.
One advantage of the present invention is that the system and method facilitates against xe2x80x9cpluggingxe2x80x9d of the burner assembly orifices and the respective liquid delivery lines feeding those burner assembly orifices. The terms, xe2x80x9cpluggingxe2x80x9d and xe2x80x9cplugxe2x80x9d, as used herein, refer to the effect of solids (formed by the chemical reactions that result from exposing liquid reactants to water, and specifically, water contained in air) that collect on the inner surfaces of the lines leading to the burner assembly and walls of the burner assembly orifices. The collected solids impede or partially impede liquid flow. The system and method of the present invention are particularly well suited for eliminating plugging, and particularly, plugging that typically occurs during periods of transient liquid flow. As used herein, the phrases, xe2x80x9cperiods of transient liquid flowxe2x80x9d and xe2x80x9ctransient liquid flow conditionsxe2x80x9d are defined as those times or periods when liquid flow is being increased to achieve steady state flow within the system. Transient liquid flow also includes these times or periods when liquid flow is being decreased from steady state liquid flow and those times or periods when liquid flow is maintained at a rate less than the selected steady state flow rate for soot deposition.
The liquid reactant preferably is prevented from being exposed to air, and thus water contained within air, during periods when the liquid reactant is not being delivered to the combustion zone to form soot. Accordingly, another advantage of the present invention is that elements previously not capable of being delivered as precursors for the formation of soot used to manufacturing glass are now capable of being delivered to form soot having qualities and properties heretofore unknown in the art. Since the liquid reactant does not come in contact with water until desired, and since the liquid precursor is not delivered as a vapor prior to its exposure to the combustion zone, elements selected from groups IA, IIA, IIIA, IIIB, IVA, IVB, VA, VB and the rare earth series of the periodic table of elements are now available to be converted by oxidation or flame-hydrolysis to soot for use in the manufacture of glass preforms.
When soot is being deposited and during other periods of steady state liquid flow, plugging is not generally a concern as the liquid enters the combustion zone before it has time to solidify enroute. However, during transient liquid flow conditions, particularly those times when liquid delivery is started and stopped, the liquid reactant flow rate is greatly reduced and the liquid reactant tends to trickle or sputter out of the liquid exit orifice of the burner assembly. During these conditions, the liquid reactant is available within the lines and within the liquid exit orifice of the burner assembly for reaction to a solid. These solids plug the orifice as well as the lines, and therefore adversely effect the soot deposition rate.
The present invention, however, delivers an evaporative liquid through the lines and burner assembly liquid exit orifice during the transient liquid flow conditions thus removing the liquid reactant from the lines and orifice during periods of reduced liquid flow rate. Because there is no liquid reactant available in the lines and orifice during this time, only the evaporative liquid is available to trickle and sputter. The evaporative liquid simply vaporizes and evaporates from the lines and orifice without leaving behind significant solids to plug the burner assembly.
To achieve these and other advantages, the system of the present invention delivers an evaporative liquid to a combustion zone through a conduit and an injector. Flow through the conduit is selectively controlled and is transitioned from the evaporative liquid to a liquid reactant. The liquid reactant is then delivered to the combustion zone through the conduit and the injector to create soot for use in the manufacture of glass. During this transitioning, a steady state flow of liquid is maintained within the conduit leading to the combustion zone. This can be achieved for example, by first delivering evaporative liquid through the conduit until a flow rate of the evaporative liquid is established which is effective to propel a uniform flow of the evaporative liquid through the conduit and burner assembly without sputtering, or dripping on or in the burner assembly, etc. Consequently, when the flow of liquid reactant is first initiated, a continuous flow rate has already been established within the conduit (i.e., by the evaporative liquid). Moreover, even though a relatively small flow rate of liquid reactant is initially present, the method of the present invention enables the liquid reactant to be propelled through the conduit at a higher speed than would otherwise occur without the aid of the evaporative liquid. When necessary, flow can be transitioned from the liquid reactant back to the evaporative liquid to purge the conduit and injector prior to terminating liquid flow to the combustion zone.
In another aspect, the invention includes a burner assembly for delivering a liquid reactant directly into a combustion zone as an aerosol to create soot for the manufacture of glass, and a dry environment positioned upstream from the burner assembly to house the liquid reactant. A conduit extending from the dry environment to the burner assembly carries the liquid reactant to the combustion zone via a flow control apparatus.
In yet another aspect, an inert gas is introduced into an enclosure to create a dry environment where a liquid reactant is staged. The enclosure includes a conduit for selectively transporting the liquid reactant to a combustion zone through an injector. An evaporative liquid is delivered through the conduit to the combustion zone and thereafter, the evaporative liquid is transitioned to the liquid reactant. The reactant is then delivered to the combustion zone through the conduit and the injector to form soot used in the manufacture of glass. The liquid reactant may then be transitioned to the evaporative liquid, and the evaporative liquid delivered to the combustion zone through the conduit and the injector to purge the system.
Additional features and advantages of the invention will be set forth in the detailed description, which follows, and in part will be apparent from the description, or may be learned by practice of the invention. It will be understood by those skilled in the art that both the foregoing general description and the following detailed description are exemplary and explanatory in nature and are intended to provide further explanation of the invention as claimed.
The accompanying drawings are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate several embodiments of the invention and together with the description serve to explain the principles of the invention.